Electron gun with auxilliary anode nearer to grid than to normal anode

ABSTRACT

A cathode ray tube is provided with an electron gun structure including a cathode, a small apertured grid, and a small apertured anode on the remote side of the grid from the cathode. In addition, a further electrode is located between the aforementioned grid and the anode. A grid electrode, disposed adjacent and preferably mounted on the grid, is provided with an aperture coaxial with the grid aperture, but this aperture has a diameter effectively several times the diameter of the grid aperture. An anode electrode is also located between the grid and the aforementioned anode and is suitably connected to approximately the same voltage as the anode. The anode electrode is similarly provided with an aperture coaxial with the other apertures, but this aperture is also several times greater in diameter than the small aperture in the grid. The anode electrode acts to strengthen the electric field between such anode electrode and the grid whereby to provide a more concentrated electron beam. The region between the anode electrode and the anode provides a divergent lensing action for producing a small electron beam spot size at the screen end of the cathode ray tube.

United States Patent [191 Odenthal [451 Mar. 25, 1975 ELECTRON GUN WITHAUXILLIARY Assig Inventor:

Filed:

Appl.

Conrad J. Odenthal, Beaverton,

Oreg.

nee:

Tektronix, lnc., Beaverton, Oreg. Jan. 5, 1973 Related U.S. ApplicationData Continuation of Ser. No. 64,938, July 31, I970,

abandoned, which is a division of Ser. No. 720,035, April 10, 1968, Pat.No. 3,567,991.

U.S. Cl. 313/449, 313/460 Int. Cl H0lj 29/02, l-lOlj 29/00 Field ofSearch 313/83, 86 US, 86 X, 85 R References Cited UNITED STATES PATENTSPrimary ExamingrRohertSegal Attorney, Agent, or Firm-Klarquist,Sparkman, Campbell, Leigh, Hall & Whinston ABSTRACT A cathode ray tubeis provided with an electron gun structure including a cathode, a smallapertured grid, and a small apertured anode on the remote side of thegrid from the cathode. In addition, a further electrode is locatedbetween the aforementioned grid and the anode. A grid electrode,disposed adjacent and preferably mounted on the grid, is provided withan aperture coaxial with the grid aperture, but this aperture has adiameter effectively several times the diameter of the grid aperture. Ananode electrode is also located between the grid and the aforementionedanode and is suitably connected to approximately the same voltage as theanode. The anode electrode is similarly provided with an aperturecoaxial with the other apertures, but this aperture is also severaltimes greater in diameter than the small aperture in the grid. The anodeelectrode acts to strengthen the electric field between such anodeelectrode and the grid whereby to provide a more concentrated electronbeam. The region between the anode electrode and the anode provides adivergent lensing action for producing a small electron beam spot sizeat the screen end of the cathode ray tube.

3 Claims, 5 Drawing Figures ELECTRON GUN WITH AUXILLIARY ANODE NEARER TOGRID THAN TO NORMAL ANODE BACKGROUND OF THE INVENTION This is acontinuation of application Ser. No. 64,938 filed July 31, 1970, nowabandoned which is a division of application Ser. No. 720,035 filed4/10/68.

In a popular form of electron gun for cathode ray tubes, an aperturedgrid electrode cooperates with an anode to produce a crossover of thetubes electron beam near the aperture in the grid. Such crossover isthen imaged on the cathode ray tube screen by the lensing action ofremaining gun electrodes. It is. of course, desired that the electrongun operate with optimum officiency whereby as large a proportion aspossible of the electrons omitted at the cathode reach the cathode raytube screen. Moreover, it is frequently desired that the image of theelectron beam crossover at the cathodev ray tube screen, that is, thespot size, be as small as possible while at the same time representing alarge beam current for producing a bright spot.

Frequently a converging electron lens is located immediately on thescreen side of the cathode ray tube guns grid. This lens tends toconcentrate the beam somewhat but it does not ordinarily have theproperty of producing a virtual image smaller than the crossover, butrather the virtual image is sometimes larger than such crossover.Because of the use of beam limiting apertures in the gun, it should benoted that requirements of high beam current and small spot size areusually contradictory. Thus, a converging lens may produce higher beamcurrent through the stopping aperture, but often has an undesired effecton spot size, while a diverging lens tends to decrease spot size but atthe same time widens the beam which has more of its current stopped outby the limiting apeerture.

SUMMARY OF THE INVENTION In accordance with the present invention, acathode ray tube includes an electron gun structure having the usualcathode, grid, and anode elements. The grid and anode are provided withsmall apertures through which the electron beam passes. A beamcrossover, or circle of least confusion, is produced near the gridaperture, while the anode accelerates an electron beam on towardthecathode ray tubes screen where such crossover is focused. The presentinvention further includes a grid electrode means and an anode electrodemeans located between the aforementioned grid and anode. The gridelectrode means is located adjacent and is preferably attached to thegrid electrode, and such grid electrode means is provided with anaperture coaxial with the grid aperture but having a diameter severaltimes the diameter of the small aperture in the grid. The anodeelectrode means, which is located closer to the anode than is the gridelectrode means, is also provided with an aperture coaxial with theother apertures, but having a diameter several times that of the gridaperture. The anode electrode means aperture is also preferably largerthan the grid electrode means aperture. The anode electrode means isconnected to a potential nearer the potential of the anode electrodethan the potential of the grid electrode, and acts to strengthen theelectric field between such anode electrode means and the gridelectrode. As a result, the strong field between the grid and the anodeelectrode means rapidly concentrates the electron beam while resultingin substantially no magnification thereof. Then, a divergent lens actionbetween the anode electrode means and the anode produces a smallervirtual image of the beam crossover for focusing upon the cathode raytube screen. It has been found that this structure produces anappreciable increase in beam current with a reduction in spot size,while cathode loading remains substantially the same.

It is accordingly an object of the present invention to provide acathode ray tube with an electron gun exhibiting improved performance,i.e. providing a smaller spot size with increased beam current andwithout substantial increase in cathode loading.

It is a further object of the present invention to provide a cathode raytube with an improved electron gun producing a stronger, moreconcentrated electron beam, wherein the effects vof space-chargespreading of the electron beam are minimized.

The subject matter which I regard as my invention is particularlypointed out and distinctly claimed in the concluding portion of thisspecification. The invention. however, both as to organization andmethod of operation, together with I further advantages and objectsthereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings whereinlike reference characters refer to like elements.

DRAWINGS FIG. I is a cross-sectional view of a portion of a prior artelectron gun;

FIG. 2 is a side elevational representation of internal structure ofacathode ray tube according to the present invention;

FIG. 3 is a cross-sectional view of a portion of an electron gunaccording to the present invention;

FIG. 4 is a plan view, partially brokenaway, of the gun portionillustrated in FIG. 3; and

FIG. 5 is a cross-sectional view of an electron gun structure accordingto an alternative embodiment of the present invention.

DETAILED DESCRIPTION Referring to FIG. 2, the internal construction of acathode ray tube according to the present invention includes a cathodel0 surrounded by grid 12 in the form of a grid cup having a smallaperture 14 through which electron beam 16 is emitted. The electron beamtravels from aperture 14 through central aperture 18 at one end of anodebarrel or first anode 20. The opposite end of first anode 20 is open andcoaxial with focus ring 22 which in turn faces the open end of secondanode 24. Second anode 24 is provided with an aperture stop 26 at theend thereof facing the screen 31 at the screen end of the cathode raytube. After passing through second anode 24, beam 16 passes between afirst pair of deflection plates 28 and a second set of orthogonallyoriented deflection plates 30, the function of which is to move theelectron beam horizontally and vertically. Typical voltages for thevarious electrodes are indicated on the drawing. Thus, the first anode20 is at a high positive voltage with respect to the cathode and grid,and therefore strongly attracts electron beam 16 which passes throughaperture 18. Grid 12, which may be biased at a voltage somewhat lowercathode 10, not only regulates the amount of current in beam 16, butalso provides a lensing action at aperture 14 for directly determiningthe size and position of an electron beam crossover. In order to producea small beam spot at the screen of the cathode ray tube, this crossoveris desirably as small as possible, i.e. the electrons are ideallyconverged to a sharply defined point. However, since the electronsemitted from cathode are nonparallel, but come off the cathode in randomdirections with varying velocities, a sharp point is not established.However, the electrons do converge into a smaller area or circle ofleast confusion normally called the crossover. This crossover is imagedon the screen of the cathode ray tube by the electron optics of thesystem.

FIG. 1 illustrates a prior art omitting portion of a commonly employedelectron gun structure, wherein like elements are referred to by thesame reference numerals used in FIG. 2. Here the electron beam crossoveris indicated at 32. Between the grid 12, which may be at zero volts orat a somewhat negative voltage, and anode 20, there is established anelectron field 34 indicated by lines substantially parallel to grid 12and anode in FIG. 1. This field may be described as a converging fieldinasmuch as the electrons of beam 16 are accelerated thereby, and tendmore in an axial direction 36 as they pass through this field. Theconvergent field therefore has the property of concentrating theelectron beam after crossover 32 so that at least a substantial portionthereof may pass through apertures 18 and 26. The converging field ofthis type ordinarily has little magnifying or demagnifying effect so faras the virtual image of the crossover is concerned, but does place thevirtual image further behind the cathode surface, and, to the focusingsection 22 of the electron gun, the electrons appear to be coming fromthis virtual image. Because of the ratio of screen distance to focussection 22 over the distance of focus 22 to the virtual image position,the virtual image is projected onto the screen 31 magnified by the ratioof these two distances. And since in the usual CRT the distance from thefocus section 22 to the screen 31 is several times larger than thedistance from focus 22 to virtual image, the spot on the screen isusually several times larger than the crossover itself.

In the region of the crossover, an appreciable spacecharge effect takesplace, caused by the close bunching of the electrons in this region.This space-charge produces spreading of the electron beam 16 in additionto the optical spreading as may take place in the electron beam beyondthe crossover or in the direction of arrow 36, thereby tending to reducebeam current passing through the aperture stop 26.

Enlargement of the beam by space-charge effects or the like can bereduced somewhat by bringing the anode 20 closer to grid 12 whereby thefield gradient of field 34 is increased, However, the grid to anodespacing in a given tube is ordinarily determined by the cathode loadingand cut-off voltage permissible or desired. Just bringing the anodecloser to the grid increases the cut-off voltage of the tube andincreases the cathode loading. Thus, for a given tube, the position ofanode 20 relative to grid 12 may be relatively fixed, depending on thecathode loading or anode voltage required.

According to the present invention. electron beam current is increased,and the spot size produced by the beam is reduced without changing thecathode loading. Thus, for a given tube, and a given amount of cathodeloading, the gun performance is enhanced. The emitting end of thecathode ray tube electron gun in accordance with the present inventionis illustrated in FIG. 3, and also the additional elements thereof areincluded in the tube structure of FIG. 2. In addition to theconventional cathode 10, grid 12, and anode 20, the structure furtherincludes a grid electrode means 38 disposed adjacent grid 12 betweengrid 12 and anode 20. The grid electrode means 38 is maintained at apotential nearer to the potential of-the grid 12 than to the potentialof anode 20. The potential of the grid electrode means is at leastbetween the potential of the grid 12 and the potential which wouldcorrespond to the position of said grid electrode means in a uniformfield between the first anode and the grid, assuming such uniform fieldexisted. The grid electrode means 38 is suitably a thin conducting metalwafer or disc of conducting material and is provided with a centralaperture 40 coaxially aligned with aperture 14 and with electron beam16. However, the diameter of aperture 40 is several times the smalldiameter of aperture 14. The thickness of the wafer comprising the gridelectrode means is suitably comparable with the diameter of aperture 14in the grid electrode.

In the illustrated embodiment, and preferably in actual practice, gridelectrode means 38 is mounted immediately upon grid 12 whereby the gridelectrode means 38 forms an annular shoulder portion surroundingaperture 14 but spaced radially therefrom. Thus, in this case, gridelectrode means 38 resides at the same potential as grid 12, i.e. asselected by potentiometer 44 connected between ground and a minus volts.Aperture 40 is suitably cylindrical and extends axially in the directionof arrow 42 to the extent of the width of the wafer or disc.

Additionally, an anode electrode means 40 is supported between grid 12and anode 20. and at least a portion thereof is desirably closer toanode 20 than is any portion of grid electrode means 38. Anode electrodemeans 46 suitably comprises a thin conducting metal disc or waferdisposed between grid electrode means 38 and anode 20. Anode electrodemeans 46 is provided with a central aperture 48 coaxially aligned withthe apertures of the other electrodes and with the electron beam 16',and has an inside diameter larger than the inside diameter of aperture14, or of aperture 18 which is comparable in size with aperture 14. Theinside diameter of aperture 48 is also preferably greater than theinside diameter of aperture 40 of grid electrode means 38. Aperture 48is also suitably cylindrical in the direction of the electron beam andarrow 42 through the width of anode electrode means 46, wherein suchwidth is comparable to the diameter of aperture 14. Although cylindricalapertures in electrode means 38 and 46 achieve the best results, it ispossible that these apertures be other than uniformly cylindrical, solong as the effective diameters of these apertures are appreciablylarger than the diameter of apertures 14 and 18.

Anode electrode means 46 is connected to a potential nearer to thepotential of the first anode 20 than the potential of the grid 12. Thepotential of anode electrode means is at least between the potential ofthe anode and the potential which would correspond to the position ofthe anode electrode means in a uniform field between the anode and thegrid, assuming such uniform field existed. Under these circumstances.the anode electrode means 46 strengthens the electric field betweenitself and grid 12, compressing the equipotontials therebetween, as canbe seen in FIG. 3. The anode electrode means 46 is most advantageouslyconnected to anode so as to reside at the same potential therewith.

Since the field is strengthened in region 50 between anode electrodemeans 46 and grid 12, or has a higher gradient therebetween, theelectron beam 16' will be more greatly accelerated in region 50 than inthe corresponding portion of field 34 in FIG. 1. Therefore, electronbeam 16' will be concentrated more densely, resulting in a narrower beamby the time it reaches field region 52. This added concentration of thebeam results from two factors. First, the greater field gradientproduces a more convergent lensing action resulting in less widening ofthe beam that would otherwise take place after crossover 32. Second, theadded acceleration of the electrons of electron beam 16 in region 50aids in overcoming space-charge spreading of the beam as would otherwiseoccur near and subsequent to crossover 32. Therefore, by the time thebeam reaches region 52 between anode electrode means 46 and anode 20,the beam is more concentrated whereby substantially more beam currentmay later pass through aperture 26 and reach the screen end of thecathode ray tube. As the beam passes through region 52, a divergencetakes place inasmuch as the curvature of the field in region 52 providesa diverging lensing action. This divergence can more easily take placein this region while the beam is at a lower potential than it subsequently reaches, e.g. at anode 20 and beyond. This divergence producesa smaller spot size on the screen end of the cathode ray tube. Thereason for the smaller spot size is related to the electron opticsinvolved between anode electrode means 46 and anode 20. The divergingproperty of the field produces a smaller virtual image of the crossover(or of the crossover image already produced by the field in region 50)which is then focused at the screen end of the tube. This smallervirtual image is produced in the same manner that a diverging opticallens produces a smaller virtual image of an object viewed therethrough.Since the beam is well concentrated before entering region 52, thisdivergence together with the action of aperture stop 26 does not detractmaterially from the beam current reaching the screen end of the tube ornearly as much as a divergence taking place after the beam has spreadfurther. The field in region 50 is considered a converging fieldalthough the field equipotentials are substantially parallel to oneanother and to grid 12 and anode 20 so far as the smaller diameterelectron beam is concerned at that point. This field narrows theelectron beam in the sense that the electron beam, because ofacceleration, does not spread as much as it otherwise would. Theconverging field formed of substantially parallel and planarequipotentials does not produce magnification, i.e. a larger virtualimage of the crossover. Rather, the virtual image produced by the fieldin region 50 is approximately the same size as the crossover, but thefield in region 52 then produces demagnification thereof.

It should be noted that the increased beam current and decreased spotsize provided by the present structure is accomplished without increasein cathode loading. It is postulated that grid electrode means 38,residing at a low potential, i.e. the potential of the grid, holds thecompressed electric field in region 50 from being forced closer toaperture 14. Thus, it will be observed that equipotential 54 in FIG. 3is no closer to aperture 14 than is corresponding equipotential 54associated with the FIG. 1 structure. Without grid electrode means 38,the field would be compressed closer towards the cathode, resulting inincreased cut-off voltage, undesirably high cathode loading, andundesirable electron optical effects. So far as the cathode isconeerned, the field drawing the electrons therefrom appears the same ineither the FIG. 3 or the FIG. 1 structures. Therefore, the increasedbeam current with decreased spot size can be achieved without excessivecathode loading or the like.

The following table illustrates improved advantages obtained in atypical gun structure constructed in accordance with the FIG. 3embodiment, as compared with one constructed in accordance with theprior art FIG. 1 arrangement:

In this table, data was taken with the first anode 20 at a positive 3.5kv with respect to the cathode. Cut-off voltage is taken as the value ofgrid voltage needed to visually extinguish the spot. Spot size in milswas measured by the shrinking raster method. 1,. is the cathode currentin milliamperes at zero grid bias. I,, is the corresponding portion ofthe cathode current which passes through stop aperture 26 to screen 3I.Gun efficiency is the current reaching the screen divided by currentleaving the cathode times 100.

In a typical embodiment, the construction of FIG. 3 had the dimensionsgiven in the following table:

TABLE II Spacing between cathode l0 and top The above dimensions aremerely given by way of specific example of a typical construction. Ingeneral, the aperture 40 in grid electrode means 38 should beapproximately 8 to IOtimes largerthan that ofaperture l4. Aperture 48should in turn be larger than aperture 40, for example, approximately1.25 to l.5 times the diameter of aperture 40. Anode electrode means 46should also be as close as possible to the grid electrode means 38without producing arcing therebetween at the voltages employed. Theaperture 18 in the anode is standard and is small, e.g. approximatelytwo or three times the diameter of the grid aperture 14.

The particular construction of FIG. 3 is subject to a number ofvariations. For example, referring to FIG. 5,

illustrating an alternative embodiment, grid electrode means 38 has anouter diameter not much greater than its inner diameter wherein theinner diameter provides the aperture 40. Grid electrode means 38 is anannular ring mounted on grid 12. Then an anode electrode means 46 isdisposed outside the outer diameter of grid electrode means 38. Anodeelectrode means 46 thus is disposed at least partially in overlappingrelation to the grid electrode, but, as will be appreciated, the samewill act to compress the electric field close to grid aperture 14without increasing cathode loading. Also, a divergent field will beprovided towards aperture 18 in anode 20. Other variations of thestructure according to the present invention will occur to those skilledin the art.

While I have shown and described preferred embodiments of my invention,it will be apparent to those skilled in the art that many changes andmodifications may be made without departing from my invention in itsbroader aspects. 1 therefore intend the appended claims to cover allsuch changes and modifications as fall within the true spirit and scopeof my invention.

I claim:

1. A. cathode ray tube in which the improvement comprises an electrongun structure including:

a cathode for emitting an electron beam,

an anode electrode supported in alignment with said cathode having ananode aperture through which at least a portion of said beam passes,

a grid electrode having a grid aperture therethrough supported betweensaid anode and said cathode in axial alignment therewith for passage ofsaid electron beam, said grid electrode having an annular shoulderportion on the side thereof opposite that facing the cathode, theshoulder portion having a 8 substantially cylindrical inner aperturespaced from said grid aperture by a distance several times the diameterof said grid aperture, said inner aperture being substantially coaxialwith the path of said electron beam, said anode aperture having adiameter of the same order of magnitude as said grid aperture, thediameter of said anode aperture being smaller than the diameter of saidinner aperture of said shoulder, and an intermediate anode electrodesupported next adjacent said grid electrode and the shoulder thereofbetween said grid electrode and said anode electrode in closer spacedrelation to said grid electrode than to said anode electrode, saidintermediate anode electrode having a large aperture therethrough inaxial alignment with the aforementioned grid and anode apertures, thediameter of said large aperture being in the range of 1.25 toapproximately 1.5 times larger than the inside diameter of the inneraperture of said annular shoulder portion while also being larger thanthe anode aperture. 2. A tube in accordance with claim 1 wherein saidshoulder portion has an axial dimension of the same order of magnitudeas the diameter of said grid aperture, and wherein said intermediateanode electrode overlaps and is at least partially in the same planewith said shoulder, the intermediate anode electrode being disposed atleast partially in surrounding relation to said shoulder.

3. A tube according to claim 1 wherein the cylindri cal inner apertureof the annular shoulder portion has a diameter at least approximately 8to 10 times the di- UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION PATENT NO. 3,873,878

DATED r h 25, 1975 |NVENTOR(S) 3 .Conrad J. Odenthal M It is certifiedthat error appears in the above-identified patent and that said LettersPatent 5 are hereby corrected as shown below:

In the Title, "AUXILLIARY" should be AUXILIARY-.

Column 1, line 16, "omitted" should be -emitted.

Column 1, line 36, "apeerture" should be -aperture.

Column 2, line 66, after "lower" insert -than-.

Column 3, line 13, "omitting" should be -emitting.

Column 4, line 35, "40" should be 46-.

. Column 5, lines 1-2, "equipotontials should be -'-equipotentialsSigned and Scaled this Twelfth D i October 1976 I [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oj'Paremsand Trademarks

1. A cathode ray tube in which the improvement comprises an electron gunstructure including: a cathode for emitting an electron beam, an anodeelectrode supported in alignment with said cathode having an anodeaperture through which at least a portion of said beam passes, a gridelectrode having a grid aperture therethrough supported between saidanode and said cathode in axial alignment therewith for passage of saidelectron beam, said grid electrode having an annular shoulder portion onthe side thereof opposite that facing the cathode, the shoulder portionhaving a substantially cylindrical inner aperture spaced from said gridaperture by a distance several times the diameter of said grid aperture,said inner aperture being substantially coaxial with the path of saidelectron beam, said anode aperture having a diameter of the same orderof magnitude as said grid aperture, the diameter of said anode aperturebeing smaller than the diameter of said inner aperture of said shoulder,and an intermediate anode electrode supported next adjacent said gridelectrode and the shoulder thereof between said grid electrode and saidanode electrode in closer spaced relation to said grid electrode than tosaid anode electrode, said intermediate anode electrode having a largeaperture therethrough in axial alignment with the aforementioned gridand anode apertures, the diameter of said large aperture being in therange of 1.25 to approximately 1.5 times larger than the inside diameterof the inner aperture of said annular shoulder portion while also beinglarger than the anode aperture.
 2. A tube in accordance with claim 1wherein said shoulder portion has an axial dimension of the same orderof magnitude as the diameter of said grid aperture, and wherein saidintermediate anode electrode overlaps and is at least partially in thesame plane with said shoulder, the intermediate anode electrode beingdisposed at least partially in surrounding relation to said shoulder. 3.A tube according to claim 1 wherein the cylindrical inner aperture ofthe annular shoulder portion has a diameter at least approximately 8 to10 times the diameter of the grid aperture.